Android studio 给标题栏加上返回按钮

public class RegisterActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_register);

        android.support.v7.app.ActionBar actionBar = getSupportActionBar();
        if(actionBar != null){
            actionBar.setHomeButtonEnabled(true);
            actionBar.setDisplayHomeAsUpEnabled(true);
        }
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
            case android.R.id.home:
                this.finish();
                return true;
        }
        return super.onOptionsItemSelected(item);
    }
}